The present invention relates to a valve lifter and a method of producing the same. More particularly, the invention is concerned with a valve lifter of the type used in automotive engines and adapted to be actuated by a valve actuating cam and a method of producing such a valve lifter.
Japanese Unexamined Patent Publication No. 218311/1984 discloses a method of producing a valve lifter having a cam-contact portion for sliding contact with a valve actuating cam and a lifter portion for actuating a valve, wherein the cam-contact portion is formed by sintering and is integrated with the lifter portion by means of a liquid phase occurring on the sintered material. Another method for producing a valve lifter is shown in Japanese Unexamined Patent Publication No. 176409/1983 in which a disk-shaped pressure-receiving plate is integrated to the end of the lifter body adjacent to the cam by friction welding. A similar method is disclosed also in Japanese Unexamined Patent Publication No. 40269/1980.
These known methods, however, do not employ any specific measure for ensuring sufficient mechanical strength regarding the lifter head portion at a time when the engine rotates at a high velocity. As a consequence, problems have been encountered such as breakdown of the valve lifter head when the valve lifter operates at high speed in an automotive engine. In order to enhance the strength of the lifter head portion while minimizing wear at the inner and outer surfaces of the lifter head, it has been proposed to cut the valve from an alloy steel which is usually used as the material for metal molds. In this case, since the valve lifter has to be cut from a rod stock of alloy steel, both the production efficiency and cost thereof are such that the cost is raised uneconomically.
Japanese Unexamined Patent Publication No. 40269/1980 discloses a method in which a valve lifter is produced from thin steel sheets by a deep drawing process. In this case, the upper surface of the head portion exhibits only a small resistance to mechanical bending force, so that an additional step is required for bonding a reinforcement or for conducting an additional surface treatment, with the results that both the production efficiency and cost thereof have been degraded.